There is mounting evidence that many regions of the brain, including the cerebellum, are involved in autism spectrum disorder (ASD). The cerebellum not only plays a critical role in skilled motor performance, but a variety of studies have implicated it in non-motor activities, including language. An allelic series of mouse Engrailed (En) mutants display both morphological defects in the cerebellum and topographic disruptions of afferent pathways. A better understanding of how En1/2 regulate cerebellum development should therefore provide insight into human cerebellum malformation syndromes with gross morphological changes, as well as diseases such ASD with more subtle morphological abnormalities but that could have significant cerebrocerebellar circuit dysfunction. An additional possible link between ASD and the En genes is RFLPs of human EN2 that are associated with autism. Also, En2 mutant mice have deficits in social interactions. A new global approach to studying the cerebellum is needed to relate how it is organized at the level of morphology, patterned gene expression and functionally specific afferent pathways, because the pattern of the folia and parasagittal gene expression reflects the topography of each afferent pathway. Furthermore, regional differences in foliation reflect functional specializations, as the mammalian cerebellum is segregated along the medial-lateral axis into a central vermis housing mainly motor circuits and surrounding hemispheres housing mainly cerebrocerebellar circuits. The En genes will be used as a genetic entry point to determine how parasagittal gene expression and foliation are patterned at the genetic and cellular levels in order to gain insight into how cerebellum circuitry is laid down. In addition, downstream effectors will be identified that mediate these processes and thus that could be candidate cerebellum disease genes. Since the basic foliation pattern and parasagittal gene expression is conserved across mammals, our findings in mice should provide a foundation for assessing human cerebellum development and disease. The Specific aims are: 1) Study how En1/2 regulate the distinct foliation patterns in the vermis and hemispheres, and also pattern parasagittal gene expression and mossy fiber topography by: i) Identifying the critical stages and cell types responsible for patterning foliation, gene expression and afferent topography by analyzing temporal and cell type specific En1/2 conditional knock-out mutants. ii) Identifying cellular processes regulated by En1/2 by examining Purkinje cell and granule cell expansion and migration, spatial gene expression and afferent development in En1/2 conditional loss- and gain-of-function mutants. 2) Identify and test candidate genes including Eph/Ephrins that have roles downstream of En1/2 in patterning the two coordinate systems and/or establishing afferent topography by: i) Identifying genes differentially expressed in En2 positive or En2 negative cells that are regulated by En1/2. ii) Altering expression of candidate genes and Eph/Ephrins in developing Purkinje cells or granule cells.